Various information recording techniques have been developed following the increase in volume of information processing in recent years. Particularly, the areal density of a HDD (hard disk drive) using the magnetic recording technique has been increasing at an annual rate of about 100%. Recently, the information recording capacity exceeding 60 GB has been required per 2.5-inch magnetic disk adapted for use in a HDD or the like. In order to satisfy such a requirement, it is necessary to realize an information recording density exceeding 100 Gbits/inch2. In order to achieve the high recording density in the magnetic disk for use in the HDD or the like, it is necessary to reduce the size of magnetic crystal grains forming a magnetic recording layer serving to record information signals, and further, to reduce the thickness of the layer. However, in the case of the conventionally commercialized magnetic disk of the in-plane magnetic recording type (also called the longitudinal magnetic recording type or the horizontal magnetic recording type), as a result of the size reduction of the magnetic crystal grains, there has arisen a thermal fluctuation phenomenon where thermal stability of recording signals is degraded due to superparamagnetism to thereby cause loss of the recording signals, which has thus become an impeding factor for the increase in recording density of the magnetic disk.
In order to solve this impeding factor, the magnetic disk of the perpendicular magnetic recording type has been proposed in recent years. In the case of the perpendicular magnetic recording type, as different from the case of the in-plane magnetic recording type, the easy magnetization axis of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the surface of a substrate. As compared with the in-plane magnetic recording type, the perpendicular magnetic recording type can suppress the thermal fluctuation phenomenon and thus is suitable for increasing the recording density. For example, Unexamined Patent Publication No. 2002-92865 (Patent Document 1) discloses a technique about a perpendicular magnetic recording medium having an underlayer, a Co-based perpendicular magnetic recording layer, and a protective layer that are formed on a substrate in the order named. Further, U.S. Pat. No. 6,468,670 Specification (Patent Document 2) discloses a perpendicular magnetic recording medium having a structure where an exchange-coupled artificial lattice film continuous layer (exchange-coupled layer) is adhered to a granular recording layer.    Patent Document 1:    Unexamined Patent Publication No. 2002-92865    Patent Document 2:    U.S. Pat. No. 6,468,670 Specification